1. Field of the Invention
The present invention relates to an image sensor for reading images printed on an image carrying paper. The present invention also relates to a contact type image reading apparatus using such an image sensor.
2. Description of the Related Art
Typically, in a conventional contact type image reading apparatus, light emitting diodes are used for the light source of the apparatus. One of the reasons for this is that the luminous energy of a light emitting diode is easily controlled. Another is that a light emitting diode has fast response to the control.
However, the luminous energy of a light emitting diode is rather small. Thus, for the conventional image reading apparatus using the light emitting diodes for its light source, it is impossible to perform a faster reading operation than is conventionally possible.
If the reading operation cannot be performed fast enough, the following inconvenience may occur. Recently, portable image readers have been widely utilized which can be held by the user and moved over an image carrying paper for scanning the images printed on the paper. If the reading speed of such a portable image reader is slow, the user has to hold the apparatus over the paper for an uncomfortably long time.
To overcome the above inconvenience, a cold-cathode tube may be used for a light source instead of light emitting diodes. As is well known in the art, a cold-cathode tube generates much greater luminous energy than a light emitting diode.
However, upon actuation, the temperature of a cold-cathode tube will rise by about 10-20.degree. C. due to heat loss caused by discharge current. Thus, as shown in FIG. 13, it takes about 3-5 minutes for the cold-cathode tube to reach thermal equilibrium after the starting-up. This means that the user has to wait for about 3-5 minutes until the luminous energy of the tube becomes stable, which is disadvantageous.
Further, the temperature of the cold-cathode tube in thermal equilibrium is affected by the temperature of the surroundings. This means that the luminous energy of light emitted by the cold-cathode tube is different when the temperature of the surroundings is different. For instance, as shown in FIG. 13, the output of the cold-cathode tube in thermal equilibrium at a temperature of 0.degree. C. is much smaller than the output at a temperature of 25 or 60.degree. C. In this way, even after the cold-cathode tube reaches its thermal equilibrium state, the output of the tube may be disadvantageously changed as the temperature of the surroundings changes.
To cope with the above problem, attempts have been made to control an inverter used for driving the cold-cathode tube. However, it has been found that this method does not work well for controlling the luminous energy of the cold-cathode tube. Thus, conventionally, it is impossible to perform a high-speed and accurate reading operation with a contact type image reading apparatus using a cold-cathode tube for its light source.